This application is based upon and claims the benefit of Japanese Patent Applications No. 11-262318 filed on Sep. 16, 1999, and No. 2000-229009 filed on Jul. 28, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an organic electroluminescent (EL) element having a laminate structure of two luminescent layers each containing an organic compound, and sandwiched between a pair of electrodes functioning as an anode and a cathode, for causing the two luminescent layers to emit light simultaneously so as to execute multi-color emission.
2. Description of the Related Art
Japanese Patent Laid-Open No. 8-78163 describes an organic EL element for multi-color emission by causing two luminescent layers to emit light simultaneously, as the organic EL element of the kind described above (hereinafter referred to as a laminated luminescent layer type) for executing multi-color emission. The organic EL element has a structure in which a hole-transporting luminescent layer (positioned on the anode side) and an electron-transporting luminescent layer (positioned on the cathode side) having mutually different emission colors are laminated with a carrier recombination region control layer (carrier block layer) interposed therebetween.
In this laminated luminescent layer type, multi-color emission is achieved when the hole-transporting luminescent layer and the electron-transporting luminescent layer emit light simultaneously, and the luminescent color (mixed color) obtained by synthesizing both light emissions becomes white.
The hole-transporting luminescent layer and the electron-transporting luminescent layer include, as a base material, a hole-transporting material and an electron-transporting material, respectively, each of which contains a fluorescent material (dopant) having a different luminescent color. Incidentally, the organic EL element uses generally those materials that are fluorescent under the solid state for both of the base material and the dopant.
The laminated luminescent layer type EL element relies on the following multi-color emission mechanism. Holes are injected into the hole-transporting luminescent layer and electrons are injected into the electron-transporting luminescent layer from the electrodes (anode and cathode). The holes and electrodes are recombined to produce excitons inside both luminescent layers through the carrier recombination region control layer interposed between the luminescent layers. The excitons impart energy to each base material or to each fluorescent material, and light is emitted.
Here, the carrier recombination region control layer has a hole blocking property, and can control migration of carriers (holes/electrons) between both luminescent layers to set the recombination region of carriers to one, or both, of the luminescent layers by changing its thickness. Therefore, when the carrier recombination region control layer has a suitable thickness, the luminescent layers having mutually different luminescent colors emit light simultaneously to achieve multi-color emission.
As a result of studies, however, it is revealed that the laminated luminescent layer type organic EL element has the following problems. One of the problems is that the luminescent color changes delicately in accordance with a quantity of current applied to the luminescent layers. This problem will be described with reference to FIGS. 13A and 13B.
When the EL element is used while luminescent brightness is changed, such as when brightness is changed depending on the environment of use such as daytime and night, the current value applied to the luminescent layers is also changed. In FIGS. 13A and 13B, symbol A indicates the fluorescent peak of the fluorescent material of the hole-transporting luminescent layer, and symbol B does the fluorescent peak of the fluorescent material of the electron-transporting luminescent layer.
As shown in FIG. 13A, it is assumed, for example, that a mixed color can be obtained under the state where the peak A has a high intensity while the other peak B has a low intensity at a high current value (e.g. 100 mA/cm2). When the current value applied to the luminescent layers is decreased (to 1 mA/cm2, for example), the intensities of the peaks A and B become reverse as shown in FIG. 13B with the result that chromaticity of the mixed color by the luminescent layers changes.
The cause may presumably be as follows. When the current value changes, the supply balance of electrons and holes collapses due to the difference in energy level between the base material and the fluorescent material. The hole-transporting luminescent layer and the electron-transporting luminescent layer have different recombination quantities of electrons and holes. Eventually, the luminescence quantity changes in each luminescent layer.
The second problem is that even when a constant DC current is applied and light emission is continued, chromaticity of the luminescent color delicately changes with time. This can be explained presumably as follows. The organic materials that constitute the luminescent layers are decomposed by the application of the current and by light emission in the course of a long time (10,000 hours, for example), and they form any barrier between the hole-transporting luminescent layer and the electron-transporting luminescent layer.
The supply balance of electrons and the holes collapses due to the presence of this barrier and eventually, the luminescence quantity in each layer changes, the same peak change phenomenon as the change shown in FIGS. 13A and 13B occurs, and chromaticity changes.
The present invention has been made in view of the above problems. An object of the present invention is to prevent a change in chromaticity resulting from a change in current value and light emission time in a laminated luminescent layer type organic EL element.
According to the present invention, an organic luminescent layer of an organic EL element is composed of a hole-transporting luminescent layer made of a hole-transporting material as a base material doped with a first fluorescent material and an electron-transporting luminescent layer made of an electron-transporting material as a base material doped with a second fluorescent material. Each of the first fluorescent material and the second fluorescent material includes at least two kinds of fluorescent materials so that an emission spectrum emitted from the hole-transporting luminescent layer approximately confirms with an emission spectrum emitted from the electron-transporting luminescent layer.
The hole-transporting luminescent layer and the electron-transporting luminescent layer emit light simultaneously and the light has a mixed color as a synthesized color from the luminescent layers. Because the emission spectrum from the hole-transporting luminescent layer approximately confirms with that from the electron-transporting luminescent layer, chromaticity is hardly changed by a change in current value and light emission time.
Preferably, in each of the hole-transporting luminescent layer and the electron-transporting luminescent layer, each florescent peak wavelength of the fluorescent materials under a solid state is approximately equal to or longer than that of the base material.
Energy of excitons produced by recombination of holes and electrons in each luminescent layer depends on the energy gap of the base material of the luminescent layer. The energy gap depends on the fluorescent peak wavelength of the base material under the solid state, and is increased as the wavelength is decreased (that is, in order of redxe2x86x92greenxe2x86x92blue).
In the present invention, because the fluorescent peak wavelength of the base material is approximately equal to or shorter than those of the fluorescent materials, the energy of the base material is larger than that of the fluorescent materials. Therefore, the energy of excitons is easily transmitted from the base material to the fluorescent materials, and simultaneous light emission of the fluorescent materials in the luminescent layers can be performed effectively.
Preferably, the hole-transporting luminescent layer directly contacts the electron-transporting luminescent layer, and fluorescent peak wavelengths of the luminescent layers are in a range of 380 nm to less than 510 nm. Accordingly, holes and electrons easily recombine with each other in the luminescent layers, and simultaneous light emission can be realized easily.